For a variety of reasons, it is sometimes necessary to surgically correct an earlier implanted medical implant (most commonly a prosthetic joint) or replace it with an entirely new medical implant. Typically, this results from either a loosening of the implant in the implant site, or the deterioration of the implant due to forces such as abrasion. Ideally, an medical implant is often formed from a high-strength material which is not only able to accommodate the various loading conditions that it may encounter, but is also non-toxic to, and otherwise biocompatible with, the human body. It is also preferable to implant the device in such a way as to enhance fixation over the long term.
A number of advances have been made to increase service life of medical implants by increasing their resistance to forces such as abrasion. The advent of oxidized zirconium, first described by Davidson in U.S. Pat. No. 5,037,438 has provided a surface with superior hardness which is also resistance to brittle fracture, galling, fretting and attack by bodily fluids. A similar advance in the area of fixation stability will address the other major source of implant failure and would represent a significant advance in implant service life.
In cases of extreme loading conditions as is often the case for artificial hips, prosthetic joints may be made from metal alloys such as titanium, zirconium, or cobalt chrome alloys. Not only are these metal alloys of sufficient strength to withstand relatively extreme loading conditions, but due to their metallic nature, a metallic porous coating typically of titanium or cobalt chrome may be secured to the metal alloy by a metallic bond. Such metallic porous coatings are useful for providing initial fixation of the implant immediately after surgery, but also serve to facilitate long-termstability by enhancing bone ingrowth and on growth.
While medical implant devices made from biocompatible metal alloys are effective, they may lack certain desirable characteristics. For example, metal alloys have poor flexibility and therefore do not tend to distribute load as evenly as would be desired. Uneven loads tend to result in a gradual loosening of the implant. As such loosening becomes more severe, revision or replacement becomes necessary. For this reason, it is desirable to design medical implants generally and prosthetic joints specifically in such a way as to improve their in vivo fixation stability.
One way this problem has historically been addressed in the past is through the use of modified surfaces for medical implants which increase surface contact area and promote bone ingrowth and ongrowth. Another more recent technique involves the use of depositing material onto the surface of an implant, the material being the emission of a plasma spray source. This is discussed in U.S. Pat. Nos. 5,807,407, 6,087,553, and 6,582,470, among others, which are incorporated by reference as though fully disclosed herein.
A promising way to form porous products involves fusing materials in such as way as to effect a porous finished material. Such approaches have been the subject of past work. Electrical discharge is one mechanism by which this has been performed, as in U.S. Pat. Nos. 5,294,769, 5,352,385, and 5,421,943. Sintered materials have also been the subject of investigation as a potential solution to the issue of fixation stability improvement through the use of porous materials which allow for tissue ingrowth and ongrowth. For example, Chowdhary in U.S. Pat. No. 5,104,410, describes a prosthesis having a metallic substrate and multiple sintered layers. The sintered layers were firmed by conventional methods of sintering, using temperatures of 1100° C. for one hour at 10−5-10−6 torr. While such sintered surface imparts desirable porosity, sintering at such extreme conditions of temperature and time fundamentally alter the nature of the substrate in undesirable ways.